1. Field of the Invention
The present invention relates to a torque detection device for a power steering by which a handle steering force in a vehicle is assisted.
2. Description of Background Art
A torque detection device for a power steering is known that includes a device wherein an input shaft and an output shaft are mounted a turnable manner and are elastically coupled to each other through a torsion bar. A torque amount is detected based on the torsion amount of the torsion bar. See, for example, Japanese Patent No. 2663454 and Japanese Patent No. 4382623.
In addition, a torque detection device for a power steering is known that includes a pair of magnetostrictive films formed at the outer periphery of a shaft so as to have magnetic anisotropy in opposite directions. An excitation coil and a detection coil surround the outer periphery of each of the magnetostrictive films, wherein variations in magnetic permeability of the pair of magnetostrictive films varied according to torsional deformation of the shaft are detected based on variations in AC resistance of the excitation coil and the detection coil, whereby the torque inputted to the shaft is detected. See, for example, Japanese Patent Laid-open No. 2009-210325.
The device described in Japanese Patent No. 2663454 has difficulties in that it needs a planetary gear mechanism for the detection of torque and its structure is complicated.
A torque sensor part (7b) in the device described in Japanese Patent No. 4382623 is so configured that a difference in the rotation angle between an input shaft (704) and an output shaft (705), or strain, generated due to a resistance of a front wheel part or the like is caused to occur in a reduced diameter part of a torsion bar (709) interconnecting the input shaft (704) and the output shaft (705). In addition, a difference in the rotation angle between a first tie rod (708) interlocked with the input shaft (704) and a body to be detected (712) interlocked with the output shaft (705) is detected by use of the ring-shaped to-be-detected body (712) provided with sensing slits or the like ranging along the circumferential direction and a sensing coil (713), and, as a variation in voltage, the detection result is outputted as a signal (torque amount) inclusive of information on the turning direction of the input shaft (704). Therefore, the device has difficulties in that it is susceptible to the influences of external factors (for example, the influences of variations in the road surface or the like, not due to the driver's steering).
The device described in Japanese Patent Laid-open No. 2009-210325 has difficulties in that, when a bending force is exerted on a shaft, the device is susceptible to the influence of the bending force, since the device adopts a magnetostrictive type torque sensor for detecting a torsional strain of the shaft.